HIGH POINT, N.C.—On his second day as a vice-presidential candidate, Rep. Paul Ryan emerged Sunday as a tough-talking sidekick and flattering biographer for Mitt Romney, playing roles that Romney, the presumptive Republican nominee, has sometimes struggled to master.

Ryan, who has frequently clashed with U.S. President Barack Obama over the size and mission of the federal government as chairman of the House Budget Committee, denounced the president’s policies as failures and his governing style as corrosive.

“President Obama came into office with hope and change,” Ryan said at a factory here. “His policies have been put in place. They are not working. They are failing us. He didn’t moderate one bit at all. So now he’s turned hope and change into attack and blame, and we’re not going to fall for it.”

Ryan’s burst of campaigning came as Democrats seized on his selection to try to define him as an extremist politician who would destroy medicare and deprive women of abortion rights.

“Congressman Ryan is a right-wing ideologue, and that is reflected in the positions that he’s taken,” said David Axelrod, a senior adviser to the Obama campaign.

“He is quite extreme — good, good person, you know, genial person — but his views are quite harsh,” Axelrod said on the program “State of the Union” on CNN.

Offering a critique that is expected to be central to the Obama team’s argument, Axelrod said that Ryan’s proposal to remake medicare would allow private insurers to peel away the healthiest seniors, leaving the federal program covering the oldest and the sickest, sending costs soaring.

The crowds at Romney campaign stops have swelled significantly since he named Ryan as his running mate, breaking records for the candidate. There were 8,000 people at an outdoor rally in Manassas, Va., on Saturday, at which a number of attendees fainted because of long lines and humidity. On Sunday, Romney’s speech inside the NASCAR Technical Institute in Mooresville drew about 4,000 people, many standing outside in a parking lot.

The two men travelled to North Carolina on the second day of a four-day bus tour that will also take them to Florida, Ohio and Wisconsin.

Romney’s wife, Ann, said that she was struck by the size of the crowds, recalling how she turned to her husband and Ryan in amazement once they reached Mooresville.

“I said they’re here because they get it. They know that America’s in trouble and these are the two guys that are going to save it.”